1. Technical Field
The present invention relates to semiconductor substrates, and to semiconductor substrate manufacturing methods.
2. Related Art
Silicon carbide (SiC), a wide bandgap semiconductor, has a bandgap at least twice as large as that of the common silicon (Si), and has attracted interest as material for high-voltage devices. Because SiC has a higher crystal forming temperature than Si, it is difficult to form a single crystal ingot by using the pulling method from the liquid phase, and the sublimation method is used to form a single crystal ingot. However, it is very difficult with the sublimation method to form a large-diameter SiC substrate having a few crystal defects. For this reason, the SiC substrates currently available in the market have a diameter of 3 to 4 inches, and are very expensive.
There are different types of SiCs, such cubical SiC (3C-SiC) and hexagonal SiCs (4H-SiC, 6H-SiC), depending on the crystalline structure. Of these, the SiC (3C-SiC) having a cubical crystal structure can be formed at relatively low temperatures, and enables direct epitaxial growth on a Si substrate. In this connection, the heteroepitaxial technique that grows 3C-SiC crystal on a Si substrate surface has been studied as a means to increase the SiC substrate diameter. However, the lattice constants of Si and 3C-SiC are 0.543 nm and 0.436 nm, respectively, and the difference is about 20%. Further, Si and 3C-SiC have coefficients of thermal expansion of 2.55×10−6 K−1 and 2.77×10−6 K−1, respectively, which account for an about 8% difference. Because of these differences in lattice constant and coefficient of thermal expansion, it is difficult with Si and 3C-SiC to obtain a high-quality epitaxial film having a few crystal defects. Further, the differences in lattice constant and coefficient of thermal expansion are also problematic, because such differences generate a large stress in the Si substrate, and cause wafer bending.
There have been studies of techniques for solving such problems. For example, JP-A-11-181567 proposes forming a masking layer on a surface of a silicon carbide growth substrate, and exposing the substrate surface through openings formed in the masking layer to cause epitaxial growth of the monocrystalline silicon carbide, the opening height being at least 21/2 of the opening width and exceeding the thickness of the monocrystalline silicon carbide formed.
However, when a monocrystalline silicon carbide film of an increased thickness is formed on the whole surface of a substrate, the stress in the substrate due to the differences in lattice constant and coefficient of thermal expansion becomes prominent, and wafer bending occurs.